The present invention generally relates to protective coating systems for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention relates to coating systems that contain rhodium aluminide-based layers as, for example, environmental coatings, bond coats, and diffusion barrier layers.
Certain components of the turbine, combustor and augmentor sections that are susceptible to damage by oxidation and hot corrosion attack are typically protected by an environmental coating and optionally a thermal barrier coating (TBC), in which case the environmental coating is termed a bond coat that in combination with the TBC forms what has been termed a TBC system. Environmental coatings and TBC bond coats are often formed of an oxidation-resistant aluminum-containing alloy or intermetallic whose aluminum content provides for the slow growth of a strong adherent continuous aluminum oxide layer (alumina scale) at elevated temperatures. This thermally grown oxide (TGO) provides protection from oxidation and hot corrosion, and in the case of a bond coat promotes a chemical bond with the TBC.
Environmental coatings and TBC bond coats in wide use include diffusion coatings that contain aluminum intermetallics (predominantly beta-phase nickel aluminide (β-NiAl) and platinum-modified nickel aluminides), and overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium, rare earth metals, and reactive metals). Overlay coatings are physically and compositionally distinguishable from diffusion coatings in that a diffusion coating significantly interacts with the substrate it protects during deposition as a result of the diffusion process to form various intermetallic and metastable phases beneath the substrate surface, whereas an overlay coating does not and as a result has a limited diffusion zone and predominantly retains its as-deposited composition, which in the case of MCrAlX is a solid solution alloy. Because TBC life depends not only on the environmental resistance but also the strength of its bond coat, bond coats capable of exhibiting higher strength have also been developed, notable examples of which are beta-phase NiAl overlay coatings. In contrast to the aforementioned MCrAlX overlay coatings, which are metallic solid solutions containing intermetallic phases, the NiAl beta phase is an intermetallic compound that exists for nickel-aluminum compositions containing about 30 to about 60 atomic percent aluminum. Examples of beta-phase NiAl overlay coatings are disclosed in commonly-assigned U.S. Pat. Nos. 5,975,852 to Nagaraj et al., 6,153,313 to Rigney et al., 6,255,001 to Darolia, 6,291,084 to Darolia et al., and 6,620,524 to Pfaendtner et al. The suitability of environmental coatings and TBC bond coats formed of NiAlPt to contain fcc gamma (γ-Ni) and the gamma-prime phase (γ′-Ni3Al) has also been considered, as disclosed in U.S. Patent Application Publication Nos. 2004/0229075 to Gleeson et al., 2006/0093801 to Darolia et al., and 2006/0093850 to Darolia et al.
Aside from use as additives in MCrAlX overlay coatings, diffusion aluminide coatings, and gamma/gamma-prime phase NiAl coatings, platinum and other platinum group metals (PGM) such as rhodium and palladium have been considered as bond coat materials. For example, commonly-assigned U.S. Pat. No. 5,427,866 to Nagaraj et al. discloses PGM-based diffusion bond coats formed by depositing and diffusing platinum, rhodium, or palladium into a substrate surface, or alternatively diffusing a PGM into an otherwise conventional bond coat material.
The above-noted coating materials contain different levels of alloying constituents (e.g., aluminum) relative to the superalloys they protect. Furthermore, superalloys contain various elements, including refractory elements, that are not present or are present in relatively small amounts in these coatings. When bond coats and environmental coatings of the type described above are deposited on superalloy substrates, solid-state diffusion occurs to some degree between the coatings and the substrates at elevated temperatures often encountered by superalloy components. This migration alters the chemical composition and microstructure of both the coating and the substrate in the vicinity of the interface, generally with deleterious results. For example, migration of aluminum out of an aluminide diffusion or overlay coating reduces its oxidation resistance, while interdiffusion with the substrate beneath the coating can result in the formation of topologically close-packed (TCP) phases that, if present at sufficiently high levels, can drastically reduce the load-carrying capability of the alloy. PGM-based bond coats are also limited by their susceptibility to interdiffusion with superalloy substrates, leading to undesirable contamination of the coating and excessive Kirchendall voiding.
In view of the above, diffusion barrier coatings have been developed and evaluated. In addition to inhibiting migration of elements between a coating and the substrate it protects, diffusion barrier coatings must also be oxidation resistant, particularly if the coating is a PGM-based coating due to the oxygen permeability of platinum group metals. Examples of diffusion barriers include ruthenium-based coatings disclosed in commonly-assigned U.S. Pat. Nos. 6,306,524 to Spitsberg et al., 6,720,088 to Zhao et al., and 6,746,782 to Zhao et al. Though the coating systems discussed above represent significant advancements in protective coating systems for high-temperature components, further improvements are desired.